Lean burn engines provide improved fuel efficiency by operating with an excess of oxygen over the amount necessary for complete combustion of the fuel. Such engines are said to run “lean” or on a “lean mixture.” However, this increase in fuel economy is offset by undesired pollution emissions, specifically in the form of oxides of nitrogen (“NOx”).
One method used to reduce NOx emissions from lean burn internal combustion engines is known as selective catalytic reduction (“SCR”). SCR, when used, for example, to reduce NOx emissions from a diesel engine, involves injecting an atomized reagent into the exhaust stream of the engine in relation to one or more selected engine operational parameters, such as exhaust gas temperature, engine rpm or engine load as measured by engine fuel flow, turbo boost pressure or exhaust NOx mass flow. The reagent/exhaust gas mixture is passed through a reactor containing a catalyst, such as, for example, activated carbon, or metals, such as platinum, vanadium or tungsten, which are capable of reducing the NOx concentration in the presence of the reagent. An SCR system of this type is disclosed in U.S. Pat. No. 5,976,475.
An aqueous urea solution is known to be an effective reagent in SCR systems for diesel engines. However, use of such an aqueous urea solution involves many disadvantages. Urea is highly corrosive and attacks mechanical components of the SCR system, such as the injectors used to inject the urea mixture into the exhaust gas stream. Urea also tends to solidify upon prolonged exposure to high temperatures, such as encountered in diesel exhaust systems. Solidified urea will accumulate in the narrow passageways and exit orifice openings typically found in injectors. Solidified urea may foul moving parts of the injector and clog any openings, rendering the injector unusable.
In addition, if the urea mixture is not finely atomized, urea deposits will form in the catalytic reactor, inhibiting the action of the catalyst and thereby reducing the SCR system effectiveness. High injection pressures are one way of minimizing the problem of insufficient atomization of the urea mixture. However, high injection pressures often result in over-penetration of the injector spray plume into the exhaust stream, causing the plume to impinge on the inner surface of the exhaust pipe opposite the injector. Over-penetration leads to inefficient use of the urea mixture and reduces the range over which the vehicle can operate with reduced NOx emissions. Only a finite amount of aqueous urea can be carried on a vehicle, and what is carried should be used efficiently to maximize vehicle range and reduce the need for frequent fill ups of the reagent.
The prior art has demonstrated the use of a pulse width modulated, solenoid actuated injector for the injection of a fine spray of urea or hydrocarbon reagents into the exhaust of a diesel engine for NOx reduction across the appropriate catalyst or for increasing the temperature in the exhaust to regenerate a particulate trap. See for example U.S. Pat. Nos. 5,605,042; 5,976,475 and 6,279,603; and commonly-owned co-pending U.S. Patent Application 2005/0235632 A1. Typically these injectors have been applied to NOx reduction in large stationary diesel engines used in power generation or to heavy duty on-road or off-road mobile diesel engines such as those used in construction equipment or refuse hauling where large quantities of reagent are required due to the large quantities of NOx emitted. Typical injection rates from an injector with a 0.012 inch orifice are 36.1 to 103.5 grams/minute (“gr/min”) of 32.5% urea solution. Smaller engines such as those used in passenger cars or light duty trucks require smaller volumes of reagent due to their lower levels of NOx mass emissions. For example injection rates of 0.5-5.0 gr/min would be typical for a light duty application using urea based selective catalytic reduction for NOx control.
Reducing the orifice size of the injector can provide some reduction in flow rates, as indicated in Table 1 (below) for an injector with a 0.006 inch exit orifice. However even these rates of 3.3-25.5 gr/min are more than is required for many light duty diesel engines. Further reduction in orifice size, while possible, may become impractical from a production perspective due to limits of orifice durability and machineability as the diameter of the exit orifice becomes less than 0.006 of an inch. From a field application perspective, orifices of less than 0.006 inches, and especially below 0.004 inches, require special provisions for filtering of reagents to prevent plugging of the fine orifice with contaminants found in commercial reagents.